The near surface region of thermoplastic materials is often chemically modified to impart such materials with unique physical properties. Examples of surface-modified materials include binding materials which are coated with an adhesive layer, barriers or membranes which have been treated to facilitate gas separation or material containment and articles coated with corrosion-resistant materials.
Quality control in the manufacture of surface-modified thermoplastic articles is typically limited by the lack of a rapid, inexpensive method for determining the efficacy of surface treatment or polymer modification. Any practically useful test must be rapid, permitting the facile detection of variations in product uniformity and properties so that immediate corrective steps can be taken to eliminate such variations without necessitating termination of the process.
Assessing, monitoring and understanding the effect that such modifiers have on the interior composition and properties of surface modified thermoplastic articles requires characterization techniques capable of distinguishing variations in elemental composition and chemical structure as a function of depth within the interior of the article. The use of surface specific techniques such as X-Ray Photoelectron Spectroscopy (XPS), Auger Electron Spectroscopy (AES), Secondary Ion Mass Spectroscopy (SIMS) and high resolution Electron Energy Loss Spectroscopy (EELS) as qualitative techniques is often limited by the small volume of material sampled by the spectrographic method. Likewise, bulk sensitive techniques such as nuclear magnetic resonance, X-ray diffraction and infrared spectroscopy typically probe too deeply into, and below the surface to effect analysis of the near surface region and may physically damage the interior region under study.
An example of a surface-modified thermoplastic article is the AIROPAK.RTM. surface-modified high density polyethylene container. AIROPAK is a registered trademark of Air Products and Chemicals, Inc., Allentown, PA. The AIROPAK container is manufactured by a blow molding process wherein a blowing gas containing about 0.1 to about 20% by volume fluorine is employed during the expansion stage of the process. Thermoplastic materials capable of being treated include polyolefin polymers and copolymers of ethylene, propylene, 1-butene, 1-pentene, 4-methyl-1-pentene, 3-methyl-1-butene and 3,3-dimethyl-1-butene. A process for making such containers is disclosed in U.S. Pat. No. 3,862,284 which is assigned to Air Products and Chemicals, Inc., Allentown, PA. Such surface treated bottles exhibit superior barrier performance to oils and other hydrocarbon liquids.
Attempts have been made to measure directly the permeability of surface-modified thermoplastic articles by exposing the surface of a barrier-coated thermoplastic article, or a sample cut therefrom, to a solution of an intensely colored or fluorescent dye, removing the article from the solution after a preset period of time and visually or instrumentally determining the degree and depth of dye permeation into the interior of the article. This method is suitable only for analysis of articles which are substantially free of interfering dark colored and/or opaque pigments which correlate with those of the colored or fluorescent dyes employed. Even when used for evaluation of articles free from interfering additives, these tests are sometimes unreliable.
Available indirect methods for testing the effectiveness of surface treatment include chemical or physical detection of active components in the barrier layers, for example, fluorine in AIROPAK containers. When fluorine is used as a surface-modifier, X-ray fluorescence, X-ray Photoelectron Spectroscopy (XPS) and combustion, followed by chemical analysis, can be utilized to analyze the surface of the article. The XPS technique employs a low energy X-ray source which dislodges core electrons of molecules or atoms residing near the surface of the specimen being analyzed thereby permitting specific analysis for elements residing on the surface of the specimen.
Other techniques for determining surface properties of thermoplastic articles include the measurement of contact angle or total reflectance. Multiple Internal Reflectance (MIR), which employs Fourier transform analysis of infrared data, is considered more reliable, but is too complex to be employed for routine determination of surface properties.
Methods for profiling the chemical structure and elemental composition of a thermoplastic article as a function of distance below the exterior surface are currently being used in the semiconductor industry. Such methods employ ion sputter etching, Angle-Resolved X-ray Photoelectron Spectroscopy and Rutherford Backscattering Spectrometry. Ion beam sputter depth profiling often results in physical and chemical damage to the sample causing particularly severe damage in non-metallic materials. Consequently, use of the technique as a means for determining chemical structure or elemental composition of thermoplastic materials as a function of distance below the exterior surface of the article is precluded. Depth resolution is often limited by effects such as primary ion knock-on and ion beam mixing which substantially hinder the ability to cleanly resolve thin layered structures within the solid.
Angle-Resolved X-ray Photoelectron Spectroscopy (ARXPS) is capable of probing the chemical structure of an article as a function of distance below the exterior surface, but only to depths of about 200 Angstroms. Thus, even though the technique maintains the integrity of the sample, the use of ARXPS technique is limited to analysis of the near-surface region. Source induced X-ray damage to the surface of the thermoplastic material can also occur during the extensive analysis time required to produce an ARXPS profile utilizing a multiple-anode X-ray source. While sample damage can be diminished by employing a monochromatic Al-K.alpha. source, the analysis is then limited to a depth of about 150 Angstroms.
Rutherford Backscattering Spectrometry (RBS) yields compositional information as a function of depth and is a useful tool for analyzing the composition of thermoplastic articles to sub-surface depths up to about 2 microns. Moreover, RBS can be operated in a low ion dose mode to minimize sample damage. However, RBS does not provide chemical structural information and is limited to the detection of particular elements within the material.
A technique known as Laser Ablation Microprobe Mass Analysis (LAMMA) has been utilized for performing compositional profiles of numerous types of materials. In LAMMA. a tightly focused laser beam of long wavelength (typically&gt;500 nm) is directed onto the sample surface. The laser power density and wavelength are sufficient to produce intense local heating and ejection of ionized material from the surface. Because the intense local heating causes considerable structural damage to the material, use of the technique is limited to elemental composition profiling. The laser wavelength and power levels utilized in LAMMA have not, to date, been amenable to chemical profiling.
Current methods for analyzing the surface of thermoplastic materials include Great Britain Patent 2,160,014 B which relates to a method comprising directing a probe beam at the surface of the article to cause a sample of material to be removed therefrom., directing a non-resonant ionizing beam of radiation at the removed sample proximate the surface, the ionizing beam having an intensity sufficient to induce non-resonant photo-ionization of the sample whereby the sample is non-selectively ionized; and subJecting the ionized sample to mass-spectrometric analysis to determine the composition of the sample. Suitable probe beams for removing a sample from the surface are provided, for example, by an electron beam, an ion beam, a gas atom beam or a laser beam having a power density in the range of 10.sup.6 to 10.sup.12 W/cm.sup.2.
Work conducted by J. T. C. Yeh. as reported in J. Vac Sci. Technol. A 4(3), May/June 1986, demonstrates that a wide variety of polymers can be ablated by short UV laser pulses. Such materials include polyethylene terephthalate, various polyimides, polymethyl methacrylate, polycarbonate, various photoresists, epoxy, human and animal hair and live tissue including cornea and cardiovascular tissues. Ablation is typically effected utilizing an excimer laser at a wavelength ranging from 193 to 351 nm. The reference further states that most polymeric materials can be etched cleanly using a laser at a wavelength of 193 nm and photon energy of 6.4 eV.
An article by Burrell. Lou and Cole, (J. Vac. Sci. Technol. A. Vol. 4, (6), Nov/Dec 1986), which is not prior art to the instant invention, suggests that laser ablation might be combined with X-ray photoelectron spectroscopy to provide compositional depth profiles of polymers such as polymethyl methacrylate which are not amenable to depth profiling by the usual ion-sputtering methods.
A need in the art exists for the development of a technique suitable for depth profiling the chemical structure and elemental composition of the interior of a thermoplastic article as well as a method for determining the depth of permeation of a modifier into the interior of a thermoplastic article which has been treated with a modifier.